Balloons mounted on the distal ends of catheters are widely used in medical treatment. The balloon may be used to widen a vessel into which the catheter is inserted or to force open a blocked vessel. The requirements for strength and size of the balloons vary widely depending on the balloon's intended use and the vessel size into which the catheter is inserted. Perhaps the most demanding applications for such balloons are in balloon angioplasty in which catheters are inserted for long distances into extremely small vessels and used to open stenoses of blood vessels by balloon inflation. These applications require extremely thin walled, high strength, relatively inelastic balloons of predictable inflation properties. Thin walls are necessary because the balloon's wall and waist thicknesses limit the minimum diameter of the distal end of the catheter and therefore determine the limits on vessel size treatable by the method and the ease of passage of the catheter through the vascular system. High strength is necessary because the balloon is used to push open a stenosis and so the thin wall must not burst under the high internal pressures necessary to accomplish this task. The balloon must have some elasticity so that the inflated diameter can be controlled, enabling the surgeon to vary the balloon's diameter as required to treat individual lesions, but that elasticity must be relatively low so that the diameter is easily controllable. Small variations in pressure must not cause wide variation in diameter.
The compliance characteristics of angioplasty balloon materials are described in U.S. Pat. No. 5,447,497, incorporated herein by reference. A variety of low-compliant materials have been employed in angioplasty balloons, including polypropylene, polyimides, polyamides, and polyesters, such as PET and PEN. Such low compliant materials can generally be fabricated into higher strength balloons than balloons made of more compliant materials. The use of low compliant materials, however, has been associated with a number of minor but undesirable problems, such as poor refold characteristics, pinhole development, difficulty in bonding to the catheter structure and high friction coefficient.
To address some of these problems a number of balloon structures have been proposed in which a layer of low compliant polymer material is coated or coextruded with an over or underlying layer of another polymer material less prone to one or more of the problems occasionally encountered with low compliant balloons. Exemplary of this approach are U.S. Pat. No. 5,270,086 (Hamlin), U.S. Pat. No. 5,195,969 (J. Wang, et al.) and U.S. Pat. No. 5,290,306 (Trotta, et al), which pertain to co-extruded structures and U.S. Pat. No. 5,490,839 (L. Wang, et. al) which pertains to coated balloon structures wherein the balloon coating imparts refold and soft pliable surface characteristics. The balloons of these references are unitary structures whose compliance and burst profiles are determined primarily by the non-compliant polymer layer, with little or no contribution by the second polymer layer. However, balloons made from coextruded tubes with soft polymer material on the top layer do provide rewrap, abrasion and puncture resistance, and reduced tracking resistance.
It is also known to prepare catheter balloon structures which include two separate concentrically arranged balloon elements mounted on a catheter. References which describe such structures include U.S. Pat. No. 4,608,984, in which an outer balloon element of a highly elastic material such as latex having a deflated circumference less than the diameter of the associated catheter is disclosed for use in refolding the inner working balloon after it has been inflated and deflated; and U.S. Pat. No. 5,447,497, U.S. Pat. No. 5,358,487 and U.S. Pat. No. 5,342,305, in which a non-linear compliance curve is obtained from two different sized balloon elements or from use of an inner balloon which bursts at some pressure below the burst pressure of the outer element. The dual concentric balloon structures, are made of materials of quite different strength characteristics and tend to give balloons whose burst strength is little different from to the burst strength of the strongest member element (typically PET or nylon).